Cancel workflows automatically when a PR is closed

[OhmSpectator]

Currently, Eden tests (and possibly other workflows) continue running even after a pull request is closed without being merged. This leads to unnecessary consumption of GitHub Actions runner time.

For example, workflow run for #4911 continued after the PR was closed: https://github.com/lf-edge/eve/actions/runs/15417561867/job/43392693755?pr=4911

It had to be canceled manually.

Suggested solution: consider adding a workflow_run or pull_request handler to detect the closed event and cancel related jobs via the GitHub API.